ON THE PALAONTOLOGY OF COUNTY DUBLIN, 
BY 
WILLIAM HELLIER BAILY, r.a.s. 
[Read February 18th, 1878. ] 
THE fossil-bearing rocks in the vicinity of Dublin, including the 
adjoining County of Wicklow, belong to the oldest series of 
formations, being of Cambrian, Silurian and Carboniferous age ; 
with the exception of a few Pleistocene deposits, containing 
marine shells in gravels of the Glacial period. 
The Cambrian fossils first claim our attention, as belonging to 
the oldest fossiliferous formation of the British Islands. From 
the Bray railway station, a walk along the shore of about a mile 
brings us to the commencement of the series of rocks forming 
Bray Head. Certain beds in the hard, sandy shales and slates 
may be seen to be covered with impressions and markings 
which were, evidently, organic; they have. been described 
under the generic name of Oldhamia, after Dr. Oldham, who 
first made them known. Two species of these remarkable fossils 
were defined by Professor Forbes: viz., O. antiqua and O. radiata, 
He considered them to belong to the Hydrozoa, and to be allied 
to Sertularian Zoophytes. Others have considered them to 
be plants, if so, they are most probably Red Algee, allied to 
the lime-secreting Corallines. They occur in both green 
and red or purple slates. On the shore, the best locality 
for O. radiata, is the “ Periwinkle Rocks,’ at Bray Point, only to 
be reached at low water ; the finely laminated ¢reen grits at this 
place being covered with their impressions, and about a mile and 
a-half further they are plentiful in certain purplish shaly beds, 
which are interstratified with thicker beds of grit, forming the 
cliffs rising from the sea at Bray Head, near the “Cable Rock.” 
Good examples of O. antiqua may be obtained from red beds near 
the same place, but more inland, a little above the footpath, round 
the Head, and just within the boundary-wall of Kilruddery 
Demesne, also at other places close to the same footpath, in cut- 
tings of the Dublin, Wicklow and Wexford railway, and in 
various places in the cliffs upon the shore, 
